The duplication of genetic information and its partitioning to progeny cells are fundamental to the survival and propagation of all eukaryotes. Many lines of evidence suggest that proto-oncogenes and tumor suppressor genes are part of the hierarchy of genes that regulate these processes. Proto-oncogenes stereotypically are positive regulators of the cell cycle and, when their oncogenic potential is activated (i.e., they become oncogenes), comprise a gain of function in the cell. In contrast, tumor suppressor genes (or recessive oncogenes) are negative regulators, and transformation is promoted through their loss of function (Perkins et al., In: Cancer: Principles and Practices of Oncology, 4th ed., DeVita et al., eds., (PA: J. B. Lippincott Co., 1993) 35-57). Many of these transforming genes for which human homologs have been discovered are set forth in Table 1, and the cellular localization of representative oncogenes is depicted in FIG. 1 (Vande Woude et al., In: Views of Cancer Research, Fortner et al., eds., (PA: J. B. Lippincott Co., 1990) 128-143; Perkins et al., In: Cancer: Principles and Practices of Oncology, 4th ed., DeVita et al., eds., (PA: J. B. Lippincott Co., 1993) 35-57).
TABLE 1 __________________________________________________________________________ Source and Properties of Oncogenes Species of RNA Tumor Virus Oncogene Origin Source Properties __________________________________________________________________________ Integral Membrane Tyrosine Kinases Susan McDonough feline sarcoma virus v-fms Cat Sarcoma From CSF 1 receptor Avian erythroblastosis virus v-erbB Chicken Sarcoma/erythroblastosis From EGF receptor HZ4 feline sarcoma virus v-kit Cat Sarcoma UR2 avian sarcoma virus v-ros Chicken Sarcoma neu Rat Neuroblastoma met Human MNNG-treated human From HGF/SF receptor osteosarcoma cell line trk Human Colon carcinoma From NGF receptor Membrane-Associated Tyrosine Kinases Rous sarcoma virus v-src Chicken Sarcoma Yamaguchi-79 sarcoma virus v-yes Chicken Sarcoma Gardner-Rasheed feline sarcoma virus v-fgr Cat Sarcoma Fujinami sarcoma virus v-fps Chicken Sarcoma Snyder-Theilen virus v-fes Cat Sarcoma Abelson murine leukemia virus v-abl Mouse Leukemia Hardy Zuckerman 2 feline sarcoma virus v-abl Cat Sarcoma Serine-Threonine Kinases Moloney murine sarcoma virus v-mos Mouse Sarcoma 3611 murine raf Mouse Sarcoma Growth Factor Families Simian sarcoma virus v-sis Monkey Glioma/fibrosarcoma B chain PDGF int-2 Mouse Mammary carcinoma Member of FGF family ks3 Human Kaposi carcinoma Member of FGF family hst Human Stomach carcinoma Member of FGF family Ras Family Harvey murine sarcoma virus v-H-ras Rat Erythroleukemia GTP binding/GTPase Kirsten murine sarcoma virus v-K-ras Rat Sarcoma GTP binding/GTPase N-ras Human DNA Various GTP binding/GTPase Nuclear Protein Family Myelocytomatosis-29 virus v-myc Chicken Carcinoma Binds DNA myelocytomatosis N-myc Human Neuroblastoma L-myc Human Small cell lung carcinoma Avian myeloblastosis virus v-myb Chicken Myeloblastosis Binds DNA FBJ murine sarcoma v-fos Mouse Osteosarcoma Binds DNA Sloan-Kettering avian sarcoma virus v-ski Chicken Carcinoma v-jun Chicken Binds DNA p53 Mouse/human Expressed at high levels Binds SV40 large T/and in transformed cells adenovirus E1B Others Reticuloendotheliosis virus, strain T v-rel Turkey Lymphatic leukemia E26 avian leukemia virus v-ets Chicken Avian erythroblastosis virus v-erbA Chicken Erthroblastosis Derived from steroid receptor fo triiodothyronine mas Human Mammary carcinoma Transmembrane protein int1 Mouse Mammary carcinoma __________________________________________________________________________
While the number of oncogenes discovered continues to increase, the number of families to which they have been assigned has not. This may be due to the limited number of assays available for oncogene detection, or it may alternately indicate that most of the oncogene families have been identified. The assignment of oncogenes to families was originally based upon their function, structure and sequence homology, or product localization, but the oncogene families appear to be taking on a new significance in the context of their participation in the cell cycle. For instance, the unrestricted proliferation of cells transformed by oncogenes provides a strong argument that their cognate proto-oncogenes and tumor suppressor genes normally function in the regulation of the cell cycle (M. Park et al., The Metabolic Basis of Inherited Disease, Vol. 1, E. R. Scriver, A. L. Beaudet, W. S. Sly, and D. Valle, Eds. (McGraw-Hill, New York, 1989), p. 251).
With respect to characterization of members of the oncogene families, recent studies of signal transduction pathways in somatic cells have linked the products of one oncogene family either directly or indirectly to the activation of members of other families. For example, the stimulation of certain growth factor receptors by their appropriate growth factor or ligand results in the association of receptors directly with the src and raf products (Morrison et al., Cell, 58, 649-657 (1989); Kypta et al., Cell, 62, 481-492 (1990)). The receptors also associate with several proteins involved in second messenger pathways (e.g., PLC.gamma. and PI3 kinase) (Coughlin et al., Science, 243, 1191-1194 (1989); Kumjian et al., Proc. Natl. Acad. Sci. USA, 86, 8232-8236 (1989); and Margolis et al., Cell, 57, 1101-1107 (1989)) as well as with a GTPase activating protein (GAP) that enhances the activity of the ras gene product. (Kaplan et al., Cell, 61, 125-133 (1990); Kazlauskas et al., Science, 247, 1578-1581 (1990)). Mitogenic stimulation of certain tyrosine kinase growth factor receptors ultimately results in specific transcriptional induction of a well-characterized series of genes, several of which are nuclear oncogenes. (Rollins et al., Adv, Cancer Res., 53, 1-32 (1989); Vogt et al., Adv. Cancer Res., 55, 1-35 (1990); Bravo R., Cell Growth & Differentiation, 1, 305-309 (1990)).
In contrast, less progress has been made in understanding how members of the diverse oncogene families elicit expression of the transformed phenotype. The fact that the members of these families function in the same or parallel pathways facilitates assigning hierarchy and determining whether a particular family is "upstream" or "downstream" in the pathway. Growth factors or, for that matter, nuclear transcription regulators are likely not proximal effectors of the transformed phenotype. Assuming that most of the oncogene families have been identified, the most probable candidates for proximal effectors would be members of the kinase oncogene families, since these proteins might modify nuclear and/or cytoskeletal proteins necessary for induction of morphological alterations associated with the neoplastic phenotype. Elucidation of this hierarchy may provide a means to develop strategies to intervene in neoplastic transformation.
Moreover, another major unanswered question concerns how the oncogenes influence the cell cycle. The entry into the S-phase and M-phase is highly regulated and this regulation has been observed in all species from yeast through man. The gene products that mediate and control this regulation are currently being characterized. For instance, the cell cycle has been intensively studied in the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe. Even though these yeasts are as evolutionarily distant from each other as they are from mammals, certain cell cycle regulators are conserved between these yeasts, not only in structure, but also in function. As an example, the CDC28 and cdc2 genes from the budding and fission yeasts, respectively, are functionally equivalent and encode a serine kinase whose targets are influenced during the cell cycle by the appearance of cyclins. These proteins, which were first discovered in clams and sea urchins were so named because of their cyclic appearance during the cell cycle.
Similarly, an activity termed maturation promoting factor (MPF) was discovered in unfertilized amphibian eggs (Masui et al., J. Exp. Zool, 177, 129-146 (1971); Smith et al., Dev. Biol., 25, 233-247 (1971)) as the activity responsible for inducing meiotic maturation, the process by which a fully grown oocyte becomes an egg capable of being fertilized (Masui et al., Int. Rev. Cytol., 57, 185-292 (1979)). MPF was subsequently found from yeast to man in all cells undergoing meiosis or mitosis and is considered the universal regulator of M-phase in eukaryotes (Kishimoto et al., Exp. Cell Res., 137, 121-126 (1982); Kishimoto et al., J. Exp. Zool., 231, 293-295 (1984); Tachibana et al., J. Cell Sci., 88, 273-282 (1987)). MPF is responsible for nuclear envelope breakdown and chromosome condensation (Lohka et al., J. Cell Biol., 98, 1222-1230 (1984); Lohka et al., J. Cell Biol., 101, 518-523 (1985); Miake-Lye et al., Cell, 41, 165-175 (1985)). Lohka et al. (Proc. Natl. Acad. Sci. USA, 85, 3009-3013 (1988)) first purified MPF, which was subsequently shown to consist of cyclins and the homologs of the yeast p34.sup.cdc2 gene product and related proteins (Gautier et al., Cell, 54, 433-439 (1988); Gautier et al., Cell, 60, 487-494 (1990)).
Thus, in just a few years, an extraordinary series of discoveries allowed characterization of the major cell cycle regulator, in species as diverse as yeast and man. However, at present, there are more questions than answers in terms of transit of cells through the cell cycle, aberrations of this process such as occur in cancer, and the role of proto-oncogenes, oncogenes and tumor suppressor genes in mediating these processes. The correlation between cell cycle regulation and the normal and aberrant roles of proto-oncogenes, tumor suppressor genes and oncogenes, respectively, in this process suggest oncogenes and mutant tumor suppressor genes are attractive targets for therapeutic intervention. Accordingly, there remains a need for techniques to identify suitable anticancer drugs as well as new and efficacious methods and pharmaceutical compositions for the treatment of cancer in mammals, particularly humans.
Thus, it is an object of the present invention to provide such a method of identifying suitable anticancer drugs and treatments, and in particular, to provide a method of identifying drugs which selectively inhibit the growth of particular cancer cells. It is another object of the present invention to provide methods of using such drugs.
These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.